Well, according to section "4.1.2 Keywords" of the CSS 2.1
Specification, vendor-specific extensions are perfectly legal.
"In CSS2.1, identifiers may begin with '-' (dash) or '_' (underscore).
Keywords and property names, beginning with -' or '_' are reserved for
vendor-specific extensions."
Therefore, although they should be avoided, browser specific attributes
are allowed and should not be marked as illegal. Moreover, even if they
are illegal, this should apply to the respective rule and should not
affect the following attributes.